This invention relates to a crystalline antibiotic salt. In particular, it relates to the oxa-.beta.-lactam antibiotic diammonium salt represented by the formula I ##STR2## wherein the chiral center of the .alpha.-carboxy-p-hydroxyphenylacetyl side chain has the D-configuration. This invention also relates to a process for preparing the crystalline diammonium salt of the above formula as the D-epimer substantially free of the L-epimer.
U.S. Pat. No. 4,138,489, issued Feb. 6, 1979, describes the 1-oxa-.beta.-lactam antibiotic diacid from which the diammonium salt of this invention is obtained. The patent teaches salts of the diacid including the sodium and potassium salts and amine salts such as the triethylammonium salt and the procaine salt. The salts of the diacid are forms of the antibiotic useful for parenteral administration of the antibiotic. As with most .beta.-lactam antibiotics, such as the penicillins and the cephalosporins, which are used parenterally, a salt form of the antibiotic is most often the pharmaceutical form which is administered. The pharmaceutical salt form is desirably a stable crystalline salt compatible with and soluble in physiological fluids.
Although pharmaceutically acceptable salts of the oxa-.beta.-lactam diacid are known, they have not been obtained in crystalline form with the desired stability. For example, the oxa-.beta.-lactam disodium salt has been obtained as an amorphous solid. Copending application Ser. No. 32,840, filed on Apr. 24, 1979, describes the crystalline oxa-.beta.-lactam sesquidisodium salt having the D-configuration which is a pharmaceutically acceptable form of the antibiotic.